Distribution of light produced by a light source can be important or even critical in some applications. The light source can be, for example but not necessarily, a light emitting diode “LED”, a filament lamp, or a gas-discharge lamp. In many applications, there is need to direct light emitted by a light source into lateral directions so that the light is substantially directed along a geometric plane or along a geometric line. FIG. 1 shows a view of a section taken from an exemplifying light guide 101 according to the prior art for modifying a light distribution pattern of a light source 102. A light guide of the kind depicted in FIG. 1 is described, for example, in the publication EP1200772. The light guide 101 can be rotationally symmetric with respect to a geometric line 120 shown in FIG. 1. The geometric line 120 is parallel with the z-axis of a coordinate system 199. The light guide 101 is made of transparent material having refractive index greater than unity. The light guide comprises a reflective surface 105 forming an apex 106 extending into the light guide as illustrated in FIG. 1. The reflective surface is shaped to provide total reflections for directing light into directions lateral with respect to an arrival direction of the light. The light guide comprises a penetrable surface 107 on the routes of the light reflected from the reflective surface. Some of the light beams radiated by the light source 102 are depicted with dashed line arrows in FIG. 1.
In many applications, it is desirable that there are no direct light propagation paths from the light source to directions whereto the light is wanted to be directed because direct light propagation paths may cause local peaks in the light distribution pattern and thus cause unpleasant glare. In the light guide shown in FIG. 1, there is a groove 121 arranged to prevent direct propagation of light to the penetrable surface 107. A wall 122 of the groove is roughened so as to provide a scattered light penetration as illustrated in FIG. 1. An inconvenience related to the light guide illustrated in FIG. 1 is that the roughened wall 122 may cause attenuation and thus reduce the amount of light directed to the desired directions.